The invention relates to an air conditioner for motor vehicles and a method for operating a motor vehicle air conditioner.
Currently, air conditioners for motor vehicles almost exclusively use the so-called cold vapor compression process with tetrafluorethane (R134a) as a refrigerant. The refrigerant circuit is comprised among other things of an evaporator, a compressor, a liquefier, and an expansion valve. An isenthalpic throttling of the refrigerant occurs in the expansion valve. An isentropic, ideal expansion can only be approached with the aid of an expansion engine; such machines have not been used for a long time in vehicle air conditioners. There are several reasons for this. For one thing, the energetic improvement which could have been achieved by means of expansion engines was relatively low and was not in any proportion with the higher cost in comparison to a simple expansion valve. For another thing, the expansion engines that were previously possible could only be controlled with a considerable increase in technical expense; a speed regulation or a control of the inlet and outlet valves would be required for this. Primarily, with the machines that were previously possible, rapid destruction or a high amount of wear during operation was to be expected, with refrigerant expanding out of the fluid.
The object of the invention is to produce an air conditioner for vehicles which, with a design cost that is only slightly higher than conventional air conditioners, is distinguished by a significant increase in performance. In addition, a method for operating an air conditioner should be disclosed, which results in a higher performance number.
The object of improving an air conditioner is attained by virtue of the fact that the air conditioner has a refrigerant circuit in which refrigerant is brought into a wet vapor state and has at least one compressor and an expansion device which is embodied as a pressure ramming machine.
The object of producing a novel method is attained according to the invention by virtue of the fact that a pressure ramming machine is integrated into a refrigerant circuit and refrigerant is brought at least partially into a wet vapor state in the pressure ramming machine. The refrigerant is expanded from a high pressure level to a low pressure level. Preferably, refrigerant is simultaneously compressed from a low pressure level to a high pressure level.
A pressure ramming machine is distinguished by a simple manufacture. The rotor can, for example, be comprised of extruded profiles and the housing parts can be milled out of aluminum or produced as diecast parts. The rotor speed, which is on an order of magnitude of e.g. 10,000 to 20,000 rpm results in the fact that no particularly high demands are placed on strength. In contrast to other expansion engines, in pressure ramming machines, no control or regulation is required, preferably not even in the inflow and the outflow of the refrigerant. Since no valves are required, the air conditioner can be operated with a high fluid content in the wet vapor zone. Furthermore, the pressure ramming machine permits operation without a drive mechanism because a pressure ramming machine can be driven exclusively by the impetus forces of the substance expanding inside it, in this instance a refrigerant, as experiments have shown. With the pressure ramming machine, an expansion can and does take place, preferably even in the wet vapor zone, which has until now been avoided in expansion engines due to their susceptibility in terms of the destruction of or wear on the moving parts, in particular the blades in expansion turbines. For this reason, even expansion engines are almost exclusively used in cold gas processes where such problems do not occur.
Advantageous embodiments of the invention constitute the subjects of the dependent claims.
Preferably, the refrigerant is carbon dioxide which in the refrigerant circuit, e.g. when used in vehicle air conditioners, is brought at least temporarily from a supercritical state into a wet vapor state during expansion, independently of the ambient temperature. This embodiment of the air conditioner according to the invention is particularly significant. The combination of carbon dioxide and the use of a pressure ramming machine results in significant advantages. With carbon dioxide as a refrigerant, greater energetic improvements and an increase in cooling performance can be achieved by an expansion engine, namely in comparison to the tetrafluorethane used previously. According to a preferred embodiment, the air conditioner according to the invention can have a transcritical or subcritical process guidance, depending on the temperature of the heat sink, i.e. the ambient temperature during operation of the air conditioner or the internal temperature during operation of the heat pump. Since the carbon dioxide has a critical point of approximately 31xc2x0 C., operating conditions can occur in which the refrigerant circuit runs in the transcritical range, with an expansion from the supercritical into the wet vapor state or runs in the subcritical range in which the refrigerant can be converted from a fluid state into a wet vapor state.
According to a preferred embodiment, the pressure ramming machine and compressor are connected to each other so that power released during the expansion process in the pressure ramming machine is used to compress the refrigerant, as a result of which the drive mechanism for the compressor can be considerably smaller in size. The cost for the pressure ramming machine is thus at least partially recouped. The expansion work is advantageously at least partially reused in the pressure ramming machine itself as compression work so that the drive work still required for the additional compressor still required is considerably lower in comparison to an operation without a pressure ramming machine. In this embodiment, the compressor section of the pressure ramming machine is correspondingly integrated into the refrigerant circuit. However, it is also possible to provide several refrigerant circuits or branched refrigerant circuits; the compressor section is then integrated into one of these circuits and the refrigerant can mix with the medium of the circuit into which the compressor section is integrated.
The compressor, which is provided as an auxiliary compressor in addition to the pressure ramming machine, is of a lower performance than a refrigerant circuit without a pressure ramming machine.
The auxiliary compressor can be connected in series or in parallel with the compressor section of the pressure ramming machine.
According to a preferred embodiment, the pressure ramming machine is driven exclusively by the impetus forces of the mass flows of refrigerant flowing through it, for which, in principle, only a very low drive power is required.
One embodiment of the method according to the invention provides for the use of carbon dioxide as a refrigerant, which can be brought into a wet vapor state in the pressure ramming machine depending on the ambient temperature.